The objective of this application is to elucidate the cellular mechanisms involved in the action of progesterone in either inhibiting or facilitating the induction of the preovulatory LH surge by estrogen. A rat model system which allows reproducible and predictable demonstration of these effects of progesterone will be established. This model will be used to investigate the possible regulation by progesterone of the concentration and subcellular distribution of receptors for estrogen and progesterone in the brain, pituitary, and uterus, and to compare the molecular properties of these receptors in different target tissues. In addition, the specificity of the antagonistic and synergistic effects of progesterone on estrogen-induced LH release will be examined. The influence of progesterone on estrogen metabolism and on the pattern of in vitro protein synthesis by brain, pituitary and uterine tissue from estrogen stimulated rats will also be studied.